Vehicle engine systems may be configured with boosting devices, such as turbochargers or superchargers, for providing a boosted air charge and increasing peak power outputs. Turbochargers include a turbine in an exhaust path of the engine that harnesses some of the available exhaust energy to drive a compressor. The hot exhaust gas flow is fed to the turbine and expands in the turbine with a release of energy, rotating a shaft coupled to the compressor. If the boosting device is a supercharger, instead of being driven by the exhaust turbine, the compressor may be driven directly or indirectly by the engine, such as via a mechanical or kinematic connection (e.g., a belt, chain, or gear), or by an electric motor (in the case of an electric supercharger). The compressor, coupled to an intake system of the engine, conveys and compresses intake air, increasing charging of cylinders of the engine. The use of a compressor allows a smaller displacement engine to provide as much power as a larger displacement engine but with additional fuel economy benefits.
An advantage of a turbocharger over a supercharger is that the turbocharger utilizes the exhaust gas energy, whereas the supercharger places a mechanical load on the engine, thereby reducing its efficiency. However, a supercharger can generate, and make available, a desired charge pressure at all times, regardless of the operating state of the engine, particularly when the supercharger is electrically driven. In contrast, difficulties are encountered in achieving an increase in power in all engine speed ranges by turbocharging. For example, a relatively severe torque drop is observed when an engine speed is undershot. That is, if the engine speed is reduced, there is a smaller exhaust gas mass flow and, therefore, a lower turbine pressure ratio. Consequently, toward lower engine speeds, a charge pressure ratio likewise decreases. This equates to a torque drop.
For example, in the presence of low charge air flow rates, a flow velocity of the charge air in the intake system falls to such an extent that the flow approaching an impeller of the compressor is impaired. As a result, a pressure increase resulting from the charge air flowing through the compressor can be realized only to a limited extent, or not at all. Rather, the charge air flow separates from the impeller blades, a partial backward flow occurs, and the compressor begins to surge. Furthermore, in transient operation of the engine, such as during a tip-in, the turbocharger may not be able to quickly meet an increased load demand from a vehicle operator. A higher charge pressure requires an acceleration of the compressor to higher rotational speeds, which is delayed as the turbine spins up.
Other attempts to increase low end torque of a turbocharged engine include shifting a surge limit of the compressor toward smaller compressor flows in order to provide charge pressures high enough to realize a satisfactory torque characteristic of the internal combustion engine even at low engine speeds and low charge air flow rates. One example approach includes staging multiple turbochargers arranged in parallel, with each turbocharger including a turbine having a relatively small turbine cross section so that the turbines are activated successively with increasing exhaust gas flow rate. In this way, a surge limit of the turbocharger compressor is shifted toward smaller charge air flows, such that, in the presence of low charge air flow rates, adequately high charge pressures can be provided in order to ensure a satisfactory torque characteristic of the engine at low engine speeds. Furthermore, the smaller turbines and the corresponding compressors can be accelerated more quickly. As another example, a plurality of turbochargers may be connected in series. By connecting two turbochargers in series, of which one turbocharger serves as a high-pressure stage and one turbocharger serves as a low-pressure stage, a compressor characteristic map can be expanded, both in the direction of smaller compressor flows and also in the direction of larger compressor flows.
Further attempts to increase a performance of a turbocharger compressor at slow or idle engine speeds and reduce response time during transient conditions include providing an auxiliary source of propellant gas or driving fluid to the compressor. One example approach is shown by Garve et al. in U.S. Pat. No. 3,462,071 A. Therein, an auxiliary propellant fluid is supplied directly to an outer portion of impeller blades of a radial compressor via a plurality of nozzles, with the amount of fluid varied based on operating conditions.
However, the inventors herein have recognized potential issues with such systems. As one example, staging multiple turbochargers in parallel or in series may increase vehicle costs and complexity. As another example, the system of U.S. Pat. No. 3,462,071 A may not be adaptable to axial compressors. Furthermore, the inventors herein have recognized that intake air may be utilized as a propellant gas without need of an auxiliary source during low engine speed conditions.
In one example, the issues described above may be addressed by a system for an internal combustion engine, comprising: an intake system for supplying charge air; a compressor arranged in the intake system, the compressor including an impeller arranged on a rotatable shaft in a compressor housing; a first shut-off element arranged in the intake system upstream of the impeller; a bypass line that branches off from the intake system upstream of the first shut-off element and opens into the intake system again between the first shut-off element and the impeller, forming a mouth region, and in which a second shut-off element is provided; a compressed air line that opens into the bypass line between the mouth region and the second shut-off element, the compressed air line coupled to a vessel that stores compressed air; and a third shut-off element arranged in the compressed air line. In this way, intake air may be provided to the impeller during smaller compressor flow conditions at an increased velocity via the bypass line.
As one example, the bypass line may form an acute angle of inclination α with respect to the shaft in the mouth region. Furthermore, in some examples, an adjustable guide device may be positioned at the mouth region to adjust the angle of inclination α. The mouth region may have a slot-like or nozzle-like form so that a velocity of the charge air is increased by flowing through the mouth region. Furthermore, the mouth region may cause an impingement of charge air on a limited segment of the impeller so that only a sub-region of the impeller interacts with the charge air. By adjusting the first shut-off element to a closed position and the second shut-off element to an open position, charge air may be supplied to the impeller via the bypass line and not directly via the intake system. Further still, the compressed air line may supply compressed air to quickly accelerate the impeller under transient conditions, such as during an abrupt increase in engine load demand (e.g., a tip-in event), by adjusting the third shut-off element to an open position. In this way, a compact and simple system is provided for efficiently adjusting charge air flow through the compressor. As a result, a map width of the compressor is increased, such as by extending a surge margin at low flow rates by providing charge air via the bypass passage. Additionally, turbo-lag is reduced by accelerating the impeller via the compressed air line. Overall, high low end engine torque and fast transient responses may be provided.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.